Optimized interferometrically modulated array source

ABSTRACT

An improved interferometric modulator permits the reduction in size of optical transmitters. In one embodiment, the optical modulator includes amplifiers or attentuators as phase modulators. In another embodiment, two outputs from a combiner are fed to the modulator, thus avoiding the requirement for an input splitter in the modulator. Light passing through the modulator may be both phase-shifted and amplified or attenuated by optical regulator sections located in the modulator. In another embodiment, the transmitter is included as a multiple-wavelength optical communications source, where individual current sources are provided to actuate a number of light sources feeding into the combiner, a processor controls the operation of each light source, and a modulator driver receives a data input signal to be encoded on the output of the source. By combining a number of modulators, a gray scale modulator may be fabricated for producing a gray scale output, rather than a conventional binary level output.

BACKGROUND

The present invention is directed generally to optical communications,and particularly to a modulated optical transmitter.

Wavelength division multiplexing (WDM) is gaining widespread use inoptical communications because, unlike conventional communicationssystems where a higher transmission capacity usually requires fastercomponents, the transmission capacity of a single optical fiber may beincreased simply by making more effective use of the availablebandwidth, without requiring the use of faster components. Also, WDMpermits signals at different wavelengths to be routed to differentdestinations.

WDM transmitters commonly use a number of independently controllable,fixed single frequency sources whose outputs are combined into a singletransmitter output. WDM transmitters typically fall into two categories,namely those in which all optical channels are capable of beingmodulated simultaneously and independently, and those where a singlemodulator is used to modulate the output of one or more lasers.Transmitters in the latter category are described as being wavelengthselectable. Wavelength selectable WDM transmitters advantageouslyrequire only a single RF modulator connection, pose less stringentrequirements on the quality of the optical output from the lightsources, and may be used with conventional packaging. Consequently, thewavelength selectable transmitter is one of the more commonly usedoptical communications sources.

An example of a wavelength selectable transmitter is disclosed in U.S.Pat. No. 5,394,489, issued to an inventor of the present application. Inan embodiment of the disclosed invention, the output from an array ofindividually actuable semiconductor lasers is combined in a combinerintegrated on the same substrate as the lasers. An amplifier amplifiesthe single output taken from the combiner, and the amplified output issubsequently modulated in a modulator.

Interferometric modulators, such as the Mach-Zehnder modulator,demonstrate several favorable characteristics such as low or zero chirp,high power handling capabilities and low insertion loss. However,interferometric modulators conventionally suffer from the disadvantageof taking up a significant amount of space. This problem is compoundedwhen an amplifier is added to the transmitter. Other types of modulator,for example the electro-absorption filter, take up less space, althoughthey do not offer the same favorable chirp, power handling and insertionloss characteristics.

Factors such as component size and system complexity are importantconsiderations in the development of optical communications systems. Inaddition, optical losses are generally reduced along with the number ofcomponents, i.e. when the system complexity is reduced. There istherefore a need to develop an improved WDM transmitter where thetransmitter is smaller, the overall transmitter complexity is reduced,and where transmitter has fewer optical losses.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a modulated opticaltransmitter. An embodiment of the invention is directed to aninterferometric modulator that include an optical amplifier or anattenuator as a phase modulation element. The modulator may includecombinations of phase shifters, amplifiers and attenuators in each armfor overall control of the output power and depth of modulation.

Other embodiments of the invention are directed to a wavelength divisionmultiplexed optical transmitter that includes a number of individuallight sources operating at independent frequencies. Outputs from each ofthe light sources are combined in an optical combiner. In one particularembodiment, the combiner has two output ports, each feeding intorespective arms of an interferometric modulator. In another embodiment,the combiner has four output ports, feeding into two parallel modulatorswhose outputs are then combined to form a gray scale signal. In anotherembodiment, the combiner has one output port feeding into a modulatorhaving an amplifier or attenuators disposed in at least one arm.

Advantages of transmitting the output from two combiner ports directlyinto respective arms of the interferometric modulator include increasingthe amount of light entering the modulator and avoiding optical lossesassociated with an input y-branch of the modulator. The resultantincrease in light transmitted by the modulator reduces the need for anamplifier integrated in the transmitter. The reduction in the number oftransmitter components also reduces fabrication and system complexity,and reduces the overall size of the device.

Advantages of including amplifier or attenuator elements in themodulator include the ability to optimize the modulator's on/off ratio,and the overall control of modulator output power.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of an embodiment of a wavelength divisionmultiplexed (WDM) transmitter on a substrate;

FIG. 2 is a block schematic of a WDM optical communications subsystemaccording to an alternative embodiment of the present invention;

FIGS. 3A and 3B are block schematics of different embodiments of theinput/output driver of the subsystem of FIG. 2.

FIG. 4 illustrates another embodiment of wavelength division multiplexedtransmitter having a modulator with optical power regulators;

FIGS. 5A-5F illustrate embodiments of an interferometric modulatorhaving an optical power regulator;

FIG. 6 illustrates a combination of modulators for generating a grayscale output; and

FIG. 7 illustrates an embodiment of an interferometric modulator havingamplifier sections.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to optical communications systemsusing wavelength division multiplexing (WDM) for increasing the capacityof optical communications channels. The present invention is believed tobe particularly well suited for use as a wavelength selectabletransmitter in WDM optical networks.

FIG. 1 illustrates a wavelength division multiplexed optical transmitter10 in accordance with one particular embodiment of the presentinvention. A number of lasers L1-L8 are integrated on a substrate 12, bygrowing, fabricating, or the like. The lasers L1-L8 operate respectivelyat different, fixed, single wavelengths λ1-λ8 to comply with therequirements of WDM. The lasers L1-L8 may be grown from a III-Vsemiconductor material, such as InP, GaAs, their alloys, or the like.For example, the active region of the lasers L1-L8 may be GaAs or InGaPconfined by higher refractive index, p- and n-doped layers of AlGaAs orAlGaP respectively.

The lasers L1-L8 may, for example, be distributed feedback (DFB) lasersfabricated on the substrate 12 with different Bragg grating periods toproduce the different wavelengths. Alternatively, the lasers L1-L8 maybe of different types, including distributed Bragg (DBR) structures orother single fixed wavelength architectures. Backup lasers may beprovided for any of the wavelengths λ1-λ8 in order to introduceredundancy.

A proposed International Telecommunications Union (ITU) standard for anoptical communications channel allocation grid has a 100 GHz channelspacing. For lasers operating at around 1550 nm, a common wavelength foroptical communications, this channel spacing corresponds to a wavelengthseparation of approximately 0.81 nm between lasers.

Waveguides W1-W8, integrated on the substrate 12 by growth, fabrication,or the like, respectively guide the outputs from lasers L1-L8 to acombiner 14. The combiner 14 combines the light from the waveguidesW1-W8 and produces two outputs into output waveguides 16 and 18.Waveguides W1-W8 and combiner 14 may be of the same construction aslasers L1-L8 and may have, for example, a waveguide composed of GaAs orInGaP confined by higher refractive index layers of AlGaAs or AlGaPrespectively.

The output waveguides 16 and 18 form two branches of an interferometricmodulator 20 similar to a Mach-Zehnder modulator or other two-branchedmodulator. The first and second output waveguides 16 and 18 respectivelyfeed into modulating elements 22 and 24 located in each branch. Themodulating elements 22 and 24 may produce a field-induced refractiveindex change using the electro-optic effect or the enhancedquantum-confined Stark effect, resulting in respective phase shifts ineach branch. The modulator 20 typically generates a high output when themodulator elements 22 and 24 control the phases of their respectivelight beams to produce constructive interference at the output waveguide26. Controlling the modulator elements 22 and 24 so that theirrespective light beams are maximally out of phase with each otherresults in a low output from the modulator 20.

An interferometric modulator 20 is typically operated in a push/pullmode. For example, in a push/pull mode of operation a high output isgenerated by the modulator 20 when similar, but oppositely polarized,control signals are applied to respective modulating elements 22 and 24.A low output is generated by the modulator 20 when the polarities of thecontrol signals are reversed. It will be appreciated that this mode ofoperation produces low chirp on the optical signals thus modulated. Ad.c. bias may be applied to the modulating elements 22 and 24 to reach aselected modulator operating point.

The two branches of the interferometer 20 are combined in an outputwaveguide 26 which is connectable to an optical communications system.An anti-reflection coating 28 may be provided on the output face of theoutput waveguide 26 to reduce optical losses.

The combiner 14 may be one of several known forms of optical couplerwhere a plurality of inputs are combined to produce two or more outputs.In one embodiment, a star-coupler is used. In this embodiment anin-plane diffraction region 30 is integrated in the substrate where thelight propagating from each of the waveguides W1-W8 freely diffracts andmixes before being output on the output waveguides 16 and 18. Astar-coupler advantageously reduces the area of the substrate 12required for combining the different wavelengths λ1-λ8.

Each output of a star-coupler generally contains approximately 1/N timesthe power fed in through one of the input waveguides, where N is thenumber of output ports. An important feature of one embodiment of theinvention implemented with a star-coupler is that the use of two outputsfrom the combiner 14 introduces an optical power to the interferometricmodulator 20 which is approximately twice the power achievable usingonly a single output from the combiner 14. Alternatively stated, the useof two outputs from the combiner 14 reduces the overall losses by 3 dB.As a result of the reduction in loss, in certain embodiments the needfor an amplifier following the combiner 14 is reduced or eliminated.

An additional advantage of using two outputs from the star-coupler isthat the need for an input y-branch is avoided. Consequently, the spacerequired for the interferometric modulator 20 is reduced. Furthermore,the transmission losses associated with the S-bends of each input branchare removed, further enhancing the optical throughput of the transmitter10.

All components L1-L8, W1-W8, 14-26 are integrable on the substrate 12and may be fabricated using conventional semiconductor fabricationtechniques including crystal growth, photolithography, thin filmdeposition, etching, or combinations of these and other techniques. Itwill be appreciated that the lasers L1-L8 may be coupled directly to thecombiner 14, without using the intervening waveguides W1-W8.

In another arrangment, the modulating elements 22 and 24 may be providedas amplifiers integrated on the substrate 12, having gain profilesencompassing the wavelengths λ1-λ8. By controlling the current passingthrough the amplifiers, the interferometric modulator 20 modulates thesignal input from the output waveguides 16 and 18 and providesadditional gain to increase the amplitude of the optical signalspropagating therethrough. The phase of light passing through theamplifier is affected by the amplifier gain. Thus, modulation of thedrive currents applied respectively to amplifier modulating elements 22and 24 results in phase modulation in each arm of the modulator 20,leading to modulation of the light at the modulator output 26. As withconventional modulator elements, amplifier modulating elements may bed.c. biased to achieve a desired level of gain in each arm.

FIG. 2 illustrates a second embodiment of a WDM optical transmitter. Alaser array 39 includes lasers L1-Ln, where n is the total number oflaser sources. Lasers L1-Ln, are independently driven by current sourcesS1-Sn through current paths C1-Cn, respectively. Operation of thecurrent sources S1-Sn is controlled by a processor 40. The processor 40also controls a thermal control unit 42 to maintain the operatingtemperature of the source 10 within a predetermined range whose limitsare determined, in part, by considerations of the temperature-inducedwavelength shift of the lasers L1-Ln.

In one mode of operating the source, the processor 40 directs one of thecurrent sources C1-Cn, for example C2, to drive its corresponding laser,L2. Light at wavelength λ2 enters the combiner 14 and is coupled intothe output waveguides 16 and 18. The light at λ2 is modulated by themodulator 20, and the modulated output transmitted through theanti-reflection layer 28 for coupling to a communications system, forexample through an optical fiber.

The data to be transmitted on the WDM output 44 enters an informationinput/output (I/O) driver 46 which receives an incoming data stream anddrives the modulator 20 to produce a modulated output carrying the inputdata. One particular embodiment of the I/O driver 46 is illustrated inFIG. 3A. The I/O driver 46 includes a data input port 50 connected to anelectrical ribbon cable 51 carrying parallel input data. The data istransferred from the input port 50 to a multiplexer 52 which multiplexesthe electronic data and feeds it to the modulator driver 48. The output54 from the multiplexer 52 may be electronically multiplexed to a singleserial OC-48 stream. The modulator driver 48 produces a driving signal56 which drives the modulator 20 to modulate the light passingtherethrough so as to produce the desired data stream on the WDM output44.

Another embodiment of the signal control driver 46 is illustrated inFIG. 3B. In this embodiment, the data to be transmitted is receivedoptically rather than electrically. The data may be transmitted throughan optical fiber 59 carrying the input data, for example in the OC-48standard format. The data are received and detected by an opticaldetector 60. The detector 60 has sufficient bandwidth to detect the highfrequencies in the incoming data stream. The signal produced by thedetector 60 is then amplified in the amplifier 62 which provides aninput signal to the modulator driver 48. A resulting modulator drivesignal 56, directed from the modulator driver 48 to the modulator 20,controls the modulator 20 so that the WDM output 44 corresponds to thedata received by the detector 60.

FIG. 4 illustrates another embodiment of WDM optical transmitter 100.Several lasers L1-L8 are integrated on a substrate 112. Each of thelasers L1-L8 is connected via an associated waveguide W1 to W8 to acombiner 114. Light from the combiner 114 is taken from a single outputwaveguide 116 and fed to an interferometric modulator 117. Theinterferometric modulator 117 includes an input y-branch 118 connectedvia waveguide arms 121 and 123 to an output y-combiner 120. Each arm 121and 123 of the modulator 117 includes an optical power regulationsection 122 and 124 as a phase modulator. The optical power regulationsections 122 and 124 may be waveguide amplifiers or waveguideattenuators. Waveguide amplifiers are preferably fabricated from thesame gain material as the gain medium employed in the lasers L1 to L8. Awaveguide attenuator may be, for example, a reverse-biased semiconductoramplifier or a section exhibiting electro-absorption.

The refractive index of an optical power regulation section dependspartly on the applied electrical control signal. Thus, modulation of thecontrol signal applied to each optical power regulator section 122 and124 results in respective changes in the level of optical powerpropagating beyond each optical power regulation section 122 and 124,along with respective shifts in phase. Each optical power regulationsection 122 and 124 may be d.c. biased by the application of a d.c. biasvoltage from a bias generator 132, so as to produce a predeterminedlevel of optical power in each arm 121 and 123. The optical powerregulation sections 122 and 124 may be biased independently, as shown,or may be biased with a common bias signal. The optical output 130 ismodulated by applying a modulation signal to the optical power regulatorsections 122 and 124 from the modulator driver 134. Where the regulatorsections 122 and 124 employ amplifiers, , modulation may be achieved byincreasing the drive current to, for example, the first regulatorsection 122, while decreasing the drive current to the second regulatorsection 124. It will be appreciated that operating an interferometricmodulator in such a push/pull configuration results in low chirpoperation and high extinction. It will also be appreciated that the biasgenerator 132 and the the modulator driver 134 may be combined into asingle drive circuit providing a modulated d.c. bias signal to eachregulator section 122 and 124.

Modulated semiconductor amplifiers have been shown to modulate light atbit rates of up to 2.5 Gb/s where sufficient light is injected into thesemiconductor amplifiers so that the gain is saturated. Thus, theinterferometric modulator 117 is suitable for operating at OC-48 datarates where most of the interest in long-haul amplified transmissioncurrently exists.

Advantages of this embodiment include a reduction in the size of thesubstrate 112, since the functions of amplification and modulation arecombined in the interferometric modulator 117, and the need for aseparate amplifier section following the modulator is thus avoided.

An additional feature of this embodiment is that the interferometricmodulator 117 can equalize the amplitudes of the optical signalscombining at the output y-combiner 120, and thus ensure maximumextinction for the "off" state. A sensor 135, positioned to detect aportion of the output 130, feeds a detector signal to a processor 136.The processor 136 checks the on/off ratio of the output 130, i.e. theratio of the optical power in the logical high state to the power of thelogical low state. If the optical power in each arm 121 and 123 notequal, then the on/off ratio is compromised. The processor 136 respondsto a detected reduction in the on/off ratio by adjusting the regulationof either or both of the regulation sections 122 and 124 so that thepower of the optical signals combined at the output y-combiner 120 areequal, thus maximizing the on/off ratio.

It will be appreciated that each of the optical power regulator sections122 and 124 may include both amplifying and attenuating portions toallow both an increase and a reduction in the power of the light in therespective modulator arms 121 and 123.

Additional embodiments of interferometric modulator 200 are illustratedin FIGS. 5A-5F. FIG. 5A illustrates an interferometric modulator 200having a first arm 202 and a second arm 206 receiving respective lightinputs from, for example, a star coupler or an input y-branch coupler,alternatively known as a 3 dB coupler. After propagating through the twoarms 202 and 206, the light is combined at a y-combiner 210, from whichthe light propagates through an output waveguide 212. The first arm 202includes a phase shifter 204 and the second arm 206 includes an opticalpower regulator 208. The optical power regulator 208 is advantageouswhen the optical power entering the two arms 202 and 206 is different.For example, the optical power regulator 208 may be an amplifier whichis useful when the optical power entering the second arm 206 is lessthan that entering the first arm 202. The amplifying regulator 208 maybe used to amplify the light in the second arm 206, thus permitting theuser to equalize the power of the light in the two arms 202 and 206.Alternatively, the optical power regulator 208 may be an attenuator,which is useful if more optical power enters the second arm 206 than thefirst arm 202. The attenuating regulator 208 may be used to reduce thepower in the second arm 206, and thus equalize the optical power in eachof the arms 202 and 206. Thus the optical power regulator 208 iseffective for maximizing the depth of modulation achievable with themodulator 200. As stated above, in addition to changing the power of thelight passing therethrough, the optical power regulator 208 may also beused to alter the light's phase. This embodiment is particularly suitedfor use where the modulator takes two outputs from a star coupler, whichis likely not to have equal optical powers in each output.

FIG. 5B illustrates another embodiment of the modulator 200 in which aphase shifter 214 is included in the second arm 206 and the first arm202 includes an optical power regulator 216. This embodiment may permitthe user to equalize the optical power produced from each arm 202 and206 irrespective of which arm has the higher power input. Also, the useof an optical power regulator 208 and 216 in each arm 206 and 202provides the user with greater control over the total optical poweroutput from the modulator 200. Thus, the optical power regulators 208and 216 may be set to produce an output 216 having a desired power. Thephase shifter 214 is used to modulate the output. It will be appreciatedthat another phase shifter 204 may be disposed in the first arm 202, asillustrated in FIG. 5C. Such an embodiment permits low chirp, push/pullmodulation by operating in tandem with the phase shifter 214, asdescribed hereinabove.

It wil be appreciated that the modulator 200 may also be provided with aphase shifter 204 in the first arm 202 while the second arm 206 isprovided with a phase shifter 214 and an optical power regulator 208, asillustrated in FIG. 5D.

Another embodiment of the modulator 200 is illustrated in FIG. 5E, wherephase shifters 204 and 214 are disposed on respective first and secondarms 202 and 206. An optical power regulator 218 is positioned in theoutput waveguide 212. In this embodiment, phase shifting effects of theregulator 218 do not affect the modulation of the signal since the powerregulation occurs after interference in the y-combiner 210.

Another embodiment of the modulator 200 is illustrated in FIG. 5F. Themodulator 200 is provided with a single phase and intensity modulator220, which both shifts the phase and regulates the intensity of lightpassing through the first waveguide 202.

Another embodiment for providing a modulated optical output isillustrated in FIG. 6. In this embodiment, a number of inputs 150 feedinto a combiner 152 having four outputs. For example, the combiner 152may be a star coupler. A first pair of outputs 154 and 156 from thecombiner 152 feed into a first interferometric modulator 158. A secondpair of outputs 160 and 162 from the combiner 152 feed into a secondinterferometric modulator 164. Each arm 166, 168, 170, and 172, markedA-D respectively, of the two modulators 158 and 164 is provided with aphase shifter/power regulator 174, 176, 178, and 180 respectively. Eachof the phase shifter/power regulators 174, 176, 178, and 180 mayindependently include a phase shifter, a power regulator or acombination of both a shifter and a regulator. The outputs 182 and 184from respective modulators 158 and 164 are combined at the y-branchcombiner 186.

The optical output 188, E, generated by this embodiment permits pulseamplitude modulation. Assume that each signal A, B, C, and D has thesame power, P, and the same phase. Thus, the output E, given by thelinear superposition of all the signals is equal to 4P, if propagationand combiner losses are ignored. If, for example, the phase of D isaltered by 180°, then signals C and D cancel each other, and the outputE has a value of 2P. If the phase of B is altered, then A cancels B.Since C cancels D, the output E has a power of zero. Thus, thisembodiment produces an output 188 having three possible levels, i.e. agray scale, and the modulator is referred to as a gray scale modulator.

The levels of the gray scale generated by the various combinations ofinputs A-D are illustrated in Table 1. In this table, a bar below thesignal indicates that the signal is out of phase by 180° from thosesignals without the bar.

                  TABLE I    ______________________________________    Output E as a function of inputs    A       B           C     D         E    ______________________________________    A       B           C     D         4P    A       B           C     D         2P    A       B           C     D         2P    A       B           C     D         2P    A       B           C     D         2P    A       B           C     D         0    A       B           C     D         0    A       B           C     D         0    A       B           C     D         0    A       B           C     D         0    A       B           C     D         0    ______________________________________

It will be appreciated that other combinations exist where three andfour signals are out of phase by 180°. However, these combinations aresimilar to those where one and none are out of phase by 180°. In otherwords, where an odd number of signals is underlined, the output is 2Pand when the signals are all underlined the output is 4P.

It will also be appreciated that for each additional interferometricmodulator added to this embodiment, where the additional modulator isfed by the combiner 152 and the output of the additional modulator iscombined with the outputs of the other modulators, then another scalelevel is added. Thus, one modulator produces two scale levels, twomodulators produce three scale levels, three modulators produce fourscale levels, and so forth.

Another embodiment for providing modulation and amplification isillustrated in FIG. 7. A substrate 250 is provided with an activechannel waveguide 252 that passes from the laser 254, through aninterferometric modulator 256 to an output waveguide region 258. Anadvantage of this approach is that the waveguide 252 may be fabricatedin a single operation, thus avoiding the necessity of fabricatingwaveguides from different materials. When a waveguide of a firstmaterial is used in the laser and a waveguide of a second material isused in the modulator, it is difficult to match the inputs and outputsof the different waveguides.

The modulator 256 is an interferometric modulator with first and secondarms 260 and 262. Modulation in each arm is provided by phase modulators264 and 266 in each arm 260 and 262 respectively. The phase modulatorsmay be phase shifters, amplifiers or attenuators, or combinationsthereof, as described above.

Amplifier regions may be provided on the substrate 200 by formingelectrodes over certain portions of the waveguide 252. For example, anelectrode 270 formed over the waveguide 252 at the input y-branch 268provides a gain region at the input section of the modulator 256.Additionally, another gain region at the output y-combiner 272 of themodulator 256 may be provided by applying a second electrode 274 overthe output y-combiner 272.

The application of a d.c. forward bias to the gain regions under theelectrodes 270 and 274 permits amplification of the signal generated bythe laser 254. Further amplification may be provided if the phasemodulators 264 and 266 include amplifiers.

Advantages of this embodiment include ease of fabrication, in that theentire waveguide, from the laser 254 to the output region 258 may befabricated in a single operation. This avoids the requirement offabricating a modulator waveguide from a material different from thatused in the waveguide in the light source. Additionally, the use ofamplifiers permits the generation of a powerful modulated optical outputsignal 276.

The waveguides of the other embodiments described herein may also befabricated in a single operation so as to be formed of the same materialas the active waveguide in the light source or sources. Thus, forexample, the transmitter 10 illustrated in FIG. 1 may be provided withwaveguides W1-W8, a combiner 14 and output waveguides 16 and 18 allformed in the same operation, and from the same material, as the gainregions of the lasers L1-L8. In addition, the modulating elements 22 and24 may be amplifiers operated in a push-pull configuration, and theoutput waveguide 26 may be operated as an amplifier. Furthermore, otherwaveguiding regions may be provided with electrodes so as to formamplifying regions. For example, the waveguides W1 to W8 may be providedwith electrodes, either individually or in combination, in order toamplify signals generated by the lasers L1 to L8.

While various examples were provided above, the present invention is notlimited to the specifics of the examples. For example, the transmitterneed not be provided with only eight lasers, but may be provided with agreater or lesser number of lasers, as required for a particularapplication. It is understood that the coupler has a sufficient numberof inputs to accept light from all the lasers present. Although theoperation of the embodiments described above has disclosed the operationof only one laser at any one time, this is not a necessary condition.Two or more lasers may operate simultaneously.

The I/O driver is not intended to be restricted to the specificembodiments illustrated. The I/O driver may, for example, receive anumber of optical signals, or a combination of electrical and opticalsignals, multiplex the received signals and direct a multiplexed drivesignal to a driver.

A gray scale modulator need receive its input from a combiner. Forexample, a gray scale modulator could receive an input from a singlewaveguide, where the single waveguide is connected to a first y-branchdivider, and the outputs from the first y-branch divider arerespectively connected to a second and a third y-branch divider toobtain four inputs to the gray scale modulator. Additional y-branchdividers may be used to feed additional interferometric modulatorsincluded the gray scale modulator.

As noted above, the present invention is applicable to opticalcommunications systems as a transmitter source. It is believed to beparticularly useful in WDM applications as a wavelength selectabletransmitter. Accordingly, the present invention should not be consideredlimited to the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims. Various modifications, equivalent processes, aswell as numerous structures to which the present invention may beapplicable will be readily apparent to those of skill in the art towhich the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

We claim:
 1. An optical transmitter, comprising:a substrate; a pluralityof selectively actuatable light sources disposed on the substrate; afirst optical combiner region disposed on the substrate, light from eachof the light sources being input to the combiner region; two outputports outputting light from the combiner region; and a firstinterferometric modulator disposed on the substrate and having a firstand second light path to receive light from the first optical combinerregion, the first light path being optically coupled at one end to oneof the two output ports and the second light path being opticallycoupled at one end to the other of the two output ports, other ends ofthe light paths being combined into a single output.
 2. An opticaltransmitter as recited in claim 1, wherein the first optical combinerregion comprises a star coupler.
 3. An optical transmitter as recited inclaim 1, wherein the first interferometric modulator comprises aMach-Zehnder modulator.
 4. An optical transmitter as recited in claim 1,wherein the first interferometric modulator comprises first and secondphase modulators disposed in the first and second light pathsrespectively.
 5. An optical transmitter as recited in claim 1, whereinthe first interferometric modulator comprises first and second opticalpower regulators disposed in the first and second light pathsrespectively.
 6. An optical transmitter as recited in claim 1, whereinthe selectively actuatable light sources are single longitudinal modelasers, each having a unique corresponding output frequency.
 7. Anoptical transmitter as recited in claim 1, further comprising a secondinterferometric modulator disposed on the substrate and opticallycoupled to third and fourth outputs from the first combiner, and asecond optical combiner region disposed on the substrate and opticallycoupled to outputs from the first and second interferometric modulators.8. An optical transmitter system, for transmitting an optical signalusing a data input signal, comprising:a multiplexed optical source,including a substrate, a plurality of selectively actuatable lightsources disposed on the substrate, an optical combiner region disposedon the substrate, each of the light sources being optically coupled asrespective inputs to the combiner region two output ports outputtinglight from the combiner region, and an interferometric modulatordisposed on the substrate and having a first and second light path toreceive light from the first optical combiner region, the first lightpath being optically coupled at one end to one of the two output portsand the second light path being optically coupled at one end to theother of the two output ports, other ends of the light paths beingcombined into a single output; a plurality of individually actuatablecurrent sources, each connected to a respective light source; aprocessor coupled to control the plurality of individually actuatablecurrent sources; and an input/output driver provided to drive theinterferometric modulator in response to the data input signal.
 9. Asystem as recited in claim 8, wherein the input/output driver comprisesan input data port provided to receive parallel electrical inputsignals, a multiplexer coupled to the input data port to multiplex theparallel electrical signals and produce a multiplexed electrical outputsignal, and a modulator driver coupled to the multiplexer and theinterferometric modulator, to receive the multiplexed electrical outputsignal and transmit a modulator drive signal to the interferometricmodulator in response to the multiplexed electrical output signal.
 10. Asystem as recited in claim 8, wherein the input/output driver comprisesa detector provided to detect an optical input signal, an amplifiercoupled to the detector to generate an amplified signal in response tothe detected optical input signal, and a modulator driver coupled to theamplifier and the interferometric modulator, to receive the amplifiedsignal and to transmit a modulator drive signal to the interferometricmodulator in response to the amplified signal.
 11. A system as recitedin claim 8, further comprising a thermal control unit coupled to themultiplexed optical source and the processor to maintain a temperatureof the multiplexed optical source within a predetermined range.
 12. Asystem as recited in claim 8, wherein the plurality of selectivelyactuatable light sources comprise a plurality of lasers, each having aunique corresponding frequency.
 13. A system as recited in claim 8,wherein the optical combiner region comprises a star coupler.
 14. Asystem as recited in claim 8, wherein the interferometric modulatorcomprises a Mach-Zehnder modulator.
 15. A system as recited in claim 14,wherein the interferometric modulator comprises first and secondamplifiers in the first and second light paths respectively.
 16. Asystem as recited in claim 15, further comprising a controller tocontrol amplification by the first and second amplifiers so as tooptimize depth of modulation of light modulated by the interferometricmodulator.
 17. A system as recited in claim 14, wherein theinterferometric modulator comprises first and second phase modulators inthe first and second light paths respectively.
 18. An opticaltransmitter system, for transmitting an optical signal using a datainput signal, comprising:a multiplexed optical source, includingasubstrate, a plurality of selectively actuatable light sources disposedon the substrate, an optical combiner region disposed on the substrate,each of the light sources being coupled as respective inputs to theoptical combiner region, and an interferometric modulator disposed onthe substrate and coupled to an output from the optical combiner region,the modulator including an optical regulator in at least one modulatorarm; a plurality of individually actuatable current sources, eachconnected to a respective light source; a processor coupled to controlthe plurality of individually actuatable current sources; and aninput/output driver provided to drive the interferometric modulator inresponse to the data input signal.
 19. A system as recited in claim 18,wherein the optical regulator comprises an optical amplifier.
 20. Asystem as recited in claim 18, wherein the optical regulator comprisesan optical attenuator.